Most carbonaceous and hydrocarbonaceous fuels contain sulfur and a major cause of air pollution is by the sulfur dioxide emanating from the fuels when burned in industrial installations.
There have been various approaches attempting to remove sulfur from the fuel, such as coal, prior to combustion. Pelcarski et al., U.S. Pat. No. 3,033,739, granted Oct. 13, 1970, sought to remove the sulfur by employing molten iron with lime as a flux; and Sass et al., U.S. Pat. No. 3,736,233, granted May 29, 1973, desulfurized the coal by employing iron oxide.
Recently fluidized bed combustion has been considered as an approach for the clean combustion of sulfur bearing coal. One aspect of the approach is flue gas desulfurization. Amongst processes which have been described for this purpose are the direct injection of dry lime/dolomite or of wet lime, or of lime slurry, into the combustion process. In the lime slurry process the solid waste causes a disposal problem and the process costs are high, whilst in the dry lime method only a fraction of the material has proved to be very reactive and once this fraction has reacted the balance reacted very slowly.
Also limiting to the widespread use of fluidized bed combustion is the economics of the use of lime as determined by the Ca/S ratio, or otherwise stated, the lime requirement. The Ca/S ratio is usually at least 2.5, and it has been considered desirous to reduce this ratio in order to provide a more economically viable process.
In one approach Moss, U.S. Pat. No. 3,807,090, granted Apr. 30, 1974, sought to burn sulfur bearing fuels in a bed of CaO and/or by MgO, and further provided iron to remove certain metals from the fuel undergoing combustion.
Lin, U.S. Pat. No. 3,781,408, discloses that CaSO.sub.4 coated fine lime particulates when heated to above 2000.degree. F., will regenerate the CaO for use in gypsum or cement, and mentions that the addition of iron oxide will increase the speed of decomposition and lower the temperature of decomposition.
Bornemann, U.S. Pat. No. 2,222,740, discloses producing sulfur dioxide from calcium sulfate in the presence of material containing iron oxide, aluminum oxide, silicon dioxide and phosphates.
While the prior art recognizes that the subsequent addition of certain metal oxides to calcium sulfate improved the generation of sulfurous oxides, the prior art sought a single catalyst for improving both the sulfurous gas absorption and desorption which could be effectively employed during combustion as well as regeneration.
The present invention provides a method for the regeneration of lime comprising heating fluidized bed particulates, which comprise calcium sulfate and iron oxide embedded therein, to a temperature of from 900.degree. C. to 1150.degree. C. in order to generate sulfurous oxide. The embedded iron oxide is present in about 0.5 to 10 percent by weight with the total calcium on a CaO basis. The method further comprises a first step of forming the iron oxide in lime by first forming an integrally adhering coating of an iron compound on the lime and then decomposing the iron compound. The regeneration is increased by 25 to 75 percent by the presence of the embedded iron oxide.
It is a principal object of this invention to provide a catalyst for increasing the reaction rate of lime used in the absorption of sulfurous oxides.
It is therefore an object of this invention to provide a method and composition for the absorption of sulfurous gases.
It is therefore an object of this invention to provide a method for fluidized bed combustion as aforesaid, wherein the novel composition contributing to the improved removal of sulfurous oxides is recycled so as to reuse in the fludized bed combustion.
It is another object of the invention to provide a method and composition as immediately aforesaid to permit the ready recycling of the active components to the fluidized bed combustion.
It is an aspect of this invention to provide a method and composition for the improved absorption of sulfurous oxides in the combustion of carbonaceous fuels.
It is still another aspect of this invention to provide a method for preparing limestone so as to improve its sulfurous oxide removal properties in the fluidized bed combustion of carbonaceous fuels.
It is still another object of this invention to provide a method and composition to reduce the sulfurous pollutants emanating from the combustion of carbonaceous fuels.
It is still a further objective of this invention to provide a method and composition as aforesaid which employ readily available materials and which is practical in operation.